1. Field of Invention
Embodiments of the invention relate generally to transfer switches. More specifically, at least one embodiment relates to an apparatus and method for preventing an electrical backfeed resulting, for example, from operation of an automatic transfer switch.
2. Discussion of Related Art
Transfer switches are employed to increase the reliability of an electrical supply to a load by allowing the load to be supplied from two or more sources. For example, a utility electrical supply (e.g., a public utility, a municipal utility, etc.) may provide one electrical supply to a load and a source of backup power (e.g., a standby/emergency generator, uninterruptible power supply, etc.) can provide a second electrical supply to the same load. The transfer switch is used to transfer the load between the two electrical supplies in the event that one of them is unavailable. With some exceptions, applicable electrical codes and national manufacturing standards generally require that the transfer switch maintain electrical isolation between the two electrical supplies for operational and safety reasons. That is, the conductors of the first electrical supply and the conductors of the second electrical supply cannot be connected to one another, even momentarily, e.g., the two electrical supplies cannot be connected to the load in parallel. A backfeed is created when phase conductors of the first electrical supply and phase conductors of the second electrical supply are connected to one another, for example, as a result of a switch failure in a transfer switch. In one failure mode, contacts in the transfer switch weld shut, that is separate contact surfaces fuse together. Contacts do weld shut and fail to operate due to arcing and/or overheating. If for example, a contact connected to the utility fails to open in the transfer switch, the generator supply will backfeed electricity to the utility when the generator is connected to the load.
To meet the requirements for electrical isolation of different electrical supplies, transfer switches (in particular, automatic transfer switches) often employ contactors, force guided relays or motorized circuit breakers to perform the switching that transfers the load from one electrical supply to another. The contactors, relays or circuit breakers are mechanically interlocked to prevent a backfeed between the various power sources connected to the transfer switch. These approaches generally result in transfer switches that are more expensive and more complex than practical for residential applications.
In another approach, control logic is used with power transfer relays in an automatic transfer switch in a residential installation. Regardless of the status of relay contacts, the control logic initiates a transfer of a load to a generator electrical supply when the logic detects a loss of voltage in a utility electrical supply.
Manual transfer switches are sometimes used as an alternative to automatic transfer switches to connect one of two electrical supplies to the load. Manual transfer switches suffer from the obvious drawback that human intervention is required to switch from one electrical supply to another electrical supply. In addition, these switches typically include a mechanical interlock to prevent different electrical supplies from being connected to one another.